Medium-speed diesel engines are used in applications where thousands of horsepower (up to 32,000) are needed. This includes propulsion engines of deep-draft, sea-going vessels, workboats operating in the inland and coastal waterways, and stand-by or continuous electrical power generation for a variety of applications including offshore drilling platforms and industrial facilities and buildings. Typically, these engines run at a speed of about 300 to 1,200 rpm.
The main lubricant for a diesel engine generally is composed of several chemical products, together with base oil of lubricating viscosity. Amongst other things, the oil should control the deposit on moving parts due to oxidation, reduce depletion of Base Number due to oxidation, and control viscosity increases due to oxidation. In addition, it should remain stable when contaminated with water, and be able to separate water easily.
A typical engine lubricating oil formulation might consist of phenate and sulfonate detergents, ashless succinimide dispersants, anti-oxidants, zinc dithiophosphates, foam inhibitors, and anti-rust agents. Sometimes, the phenate and sulfonate detergents have been replaced with salicylates to improve performance.
Because of the relatively high cost of salicylates versus phenates, it is desirable to develop a less expensive alternative to salicylates that give better performance than current phenates.
The modification of phenates with carboxylic acids or derivatives is taught in U.S. Pat. Nos. 5,714,443; 5,716,914; and 5,728,657.
U.S. Pat. No. 5,716,912 discloses a polyalkylene succinimide formed by reaction of a mixture of an alkenyl or alkylsuccinic acid derivative, an unsaturated acidic reagent copolymer of an unsaturated acidic reagent and an olefin, and a polyamine.
U.S. Pat. No. 4,948,522 discloses use of zinc dialkyldithiophosphates derived from mixtures of primary and secondary alcohols for marine applications. WO Application 96/20265 discloses use of physical mixtures of primary and secondary zinc dithiophosphates in motor car engine oils.